The invention relates to steam boiler operation in general, and more particularly to operation of the boiler feedpump circulating deaerated feedwater from the liquid phase of the deaerator to the heat exchangers contributing to the generation of superheated steam, for a steam turbine for instance.
The boiler feedpump stands directly in the hydraulic line behind the deaerator, e.g. where finely divided feedwater from the condenser experiences degassing in the vapor phase above the tank where degassed water accumulates.
The primary function of the deaerator is to remove non-condensible gases such as oxygen and carbon dioxide from the condenser feedwater. Secondary, it heats the feedwater to its saturation point, depending upon the operating pressure of the boiler unit. This is accomplished by utilizing as auxiliary energy sources the low pressure storage tank, the economizer, pegging steam from the high pressure steam drum and/or extraction steam from the steam turbine. Thirdly, the deaerator stores feedwater and returns condensate at boiling temperature to satisfy boiler demands. The heated and deaerated water, after it leaves the deaeration section, passes to the tank or storage section where it is blanketed by a steam cushion to retain its heat. The deaerator is state of the art. A reference can be found in "Planning Fundamentals of Thermal Power Plants"/F. S. Aschner - John Wiley 1978, pp. (29-132).
The operation of the deaerator has an incidence on the boiler feedpump which extracts feedwater from the storage tank of the deaerator and feeds it to the economizer. The boiler feedpump design and operation are described on pages 137-146 of the F. S. Aschner book. One general problem with a pump is to maintain at its input a minimum pressure on the suction branch thereof. Translated into a column of water, this requirement is called the "maximum suction head" of the pump, which is determined by the "net positive suction head" (NPSH) known as the gauge reading in meters at the pump section branch minus the vapor pressure in meters, H.sub.vap, corresponding to the temperature of the liquid plus velocity head (c.sup.2 /2 g) at the pump suction. When boiling liquids are pumped from a closed vessel, the NPSH is the static liquid head in the vessel above the pump center line minus losses of head in the suction pipe (see in F. S. Aschner book page 141). The admissible suction head H.sub.s (in meters) is: EQU H.sub.s .ltoreq.H.sub.bar -H.sub.vap -H.sub.frp -H.sub.p,u -(C.sup.2 /2 g)-(safety margin against cavitation) (1)
where H.sub.bar is the atmospheric pressure in meters of the liquid; H.sub.frp is the friction resistance of the suction pipe and H.sub.pu is the inlet resistance of the pump.
Cavitation is a mechanical wearing away of the metal surface of the pump caused by the liquid leaving the guiding surface, creating a vacuum liberating air and water and forming a bubble. The bubble collapses under the high pressure of the water and a very large local pressure on the entrapped air ensues, which hits the metal surface. Pitting, vibration and noise accompany such repeated occurrence of cavitation in the pump. Cavitation also adversely affects the performance of a pump. Critical to the generation of cavitation is the minimum pressure required, namely the net positive suction head as explained in "Design and Performance of Centrifugal and Axial Flow Pumps and Compressors" by Andre Kovats, Pergamon Press, 1964, pp. 69-74, or in "Pumps and Blowers" by A. J. Stepanoff, John Wiley & Sons, 1965, pp. 3-55. Stepanoff also considers the thermodynamics of boiler feedpumps on pages 104-110 of his book.